Organophosphorus insecticides currently in wide use are biochemically reactive chemicals and can form various stable covalent bonds with macromolecule of animal cells. In view of the demonstrated side effects of many common organophosphates, potential alterations of macromolcules of vital functions by organophosphates will be studied as a possible basis of health hazards. Using radiolabelled organophosphates, covalent bond formations with various tissues and subcellular organelles will be studied in experimental vertebrates in vivo and in vitro. Intra-tissue and intra-cellular factors that influnce the macromolecular interactions of organophosphates will be examined with emphases on fate of dialkylphosphoryl moiety and desulfurated sulfur of phosphorothionates.